Topbar fixes

What does this MR do?

• Search in Navbar: Do not open full result page on enter; increased search results to 50 per thing
  • closes #315 (closed)
• Groups in Navbar: Collapsed group names navigatable
  • closes #314 (closed)
• Makes login button a button and assigns names to input fields to enable autofill (at least in Firefox)

How confident are you it won't break things if deployed?

Manually checked

Links to related issues

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))